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An Important Natural Genetic Resource of Oreochromis Niloticus (Linnaeus, 1758) Threatened by Aquaculture Activities in Loboi Drainage, Kenya

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An Important Natural Genetic Resource of Oreochromis Niloticus (Linnaeus, 1758) Threatened by Aquaculture Activities in Loboi Drainage, Kenya An Important Natural Genetic Resource of Oreochromis niloticus (Linnaeus, 1758) Threatened by Aquaculture Activities in Loboi Drainage, Kenya Titus Chemandwa Ndiwa1,2, Dorothy Wanja Nyingi1*, Jean-Franc¸ois Agnese2 1 Kenya Wetlands Biodiversity Research Group, Ichthyology Section, National Museums of Kenya, Nairobi, Kenya, 2 De´partement Conservation et Domestication, UMR IRD 226 CNRS 5554, Institut des Science de l’Evolution, Universite´ de Montpellier 2, Montpellier, France Abstract The need to improve food security in Africa through culture of tilapias has led to transfer of different species from their natural ranges causing negative impacts on wild fish genetic resources. Loboi swamp in Kenya is fed by three hot springs: Lake Bogoria Hotel, Chelaba and Turtle Springs, hosting natural populations of Oreochromis niloticus. The present study aimed at better genetic characterization of these threatened populations. Partial mtDNA sequences of the D-loop region and variations at 16 microsatellite loci were assessed in the three hot spring populations and compared with three other natural populations of O. niloticus in the region. Results obtained indicated that the hot spring populations had mitochondrial and nuclear genetic variability similar to or higher than the large closely related populations. This may be attributed to the perennial nature of the hot springs, which do not depend on rainfall but rather receive permanent water supply from deep aquifers. The study also revealed that gene flow between the three different hot spring populations was sufficiently low thus allowing their differentiation. This differentiation was unexpected considering the very close proximity of the springs to each other. It is possible that the swamp creates a barrier to free movement of fish from one spring to the other thereby diminishing gene flow. Finally, the most surprising and worrying results were that the three hot spring populations are introgressed by mtDNA genes of O. leucostictus, while microsatellite analysis suggested that some nuclear genes may also have crossed the species barrier. It is very likely that the recent intensification of aquaculture activities in the Loboi drainage may be responsible for these introgressions. Taking into account the importance of these new genetic resources, protection and management actions of the Loboi swamp should be accorded top priority to prevent the loss of these spring populations. Citation: Ndiwa TC, Nyingi DW, Agnese J-F (2014) An Important Natural Genetic Resource of Oreochromis niloticus (Linnaeus, 1758) Threatened by Aquaculture Activities in Loboi Drainage, Kenya. PLoS ONE 9(9): e106972. doi:10.1371/journal.pone.0106972 Editor: Valerio Ketmaier, Institute of Biochemistry and Biology, Germany Received May 22, 2014; Accepted August 4, 2014; Published September 15, 2014 Copyright: ß 2014 Ndiwa et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: The authors confirm that all data underlying the findings are fully available without restriction. All relevant data are within the paper and its supporting information files. mtDNA sequences are stored at Genbank (accession numbers provided in Table 1). Funding: TCN received a personal grant from IRD (Institut de Recherche pour le De´veloppement) and MAE (French Ministry of Foreign Affairs). Field work was funded by Kenya Wetlands Research and National Museums of Kenya (NMK). Laboratory work was funded 50% by IRD and 50% by NMK. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * Email: [email protected] Introduction production estimated at 2,790,350 metric tonnes in 2011, and valued at 4.52 billion USD [10]. This species is favoured by fish Tilapias are some of the most important species for fisheries and farmers due to its fast growth rate and its ability to tolerate a wide aquaculture in Africa. This has contributed to their massive range of environmental conditions [11]. Resistance to diseases and transfer not only within Africa, but to other countries around the good consumer acceptance has further promoted its culture world [1,2,3]. These transfers are a major concern because of the worldwide [12]. invasive nature of tilapia species and their ability to hybridize with Seven subspecies of Nile tilapia have been described based on local species [3,4,5]. Introgression of alien genes can leads to morphological characteristics [8]: O. n. niloticus from West-Africa disruption of specific allele combinations responsible for adapta- and River Nile, O. n. baringoensis from Lake Baringo, O. n. tion of the populations to their environments hence reducing their sugutae from River Suguta (Kenya), O. n. eduardianus from Lakes fitness [6,7]. Edward, Albert, George and Tanganyika, O. n. vulcani from Lake Within tilapias, Oreochromis niloticus (Linnaeus, 1758), the Nile Turkana, O. n. cancellatus from River Awash, and Lake Tana and tilapia, is the most economically important species, with a wide O. n. filoa from hot springs in the Awash system. Recently, Nyingi natural distribution in Africa. Its natural range covers the entire et al. [5] discovered a natural population from Lake Bogoria Hotel Nilo-Sudanian province (Senegal to Nile), Ethiopian Rift-Valley Spring (Kenya). This spring that drains into Loboi swamp is province, Kivu province, North Tanganyika Province (Ruzizi) and characterized by an elevated water temperature (36uC) and pH the Northern part of the East African Rift-Valley [8,9]. Currently, ranging from 6.4–6.9 [13]. This population of O. niloticus was Nile tilapia is cultured in more than 100 countries and its characterised by 10 private microsatellite alleles and five private PLOS ONE | www.plosone.org 1 September 2014 | Volume 9 | Issue 9 | e106972 Introgressed Oreochromis niloticus in Loboi Swamp mtDNA haplotypes. Both extents of mitochondrial and microsat- compared, three from the hot springs of the Loboi drainage ellite differentiations were in the range of those observed among (Turtle Spring, Chelaba Spring and Lake Bogoria Hotel Spring) other naturally occurring discrete populations in the region (Lakes (Figures S1–S3), while other populations were from Kenyan Lakes Baringo and Turkana, and River Suguta). These observations or Rivers (Baringo, Turkana, Suguta). Authorization to collect and indicated that the Lake Bogoria Hotel Spring population was not sacrifice fish within the protected areas in Lake Turkana was as earlier hypothesised, introduced from other neighbouring East provided by the means of a permit (number KWS/BRM/5001). African natural populations, but represents a previously unknown All fishing and processing of tissue and whole fish specimens was natural population. carried out under existing collaborative arrangements with the This population offers new opportunities for aquaculture due to Kenya Wildlife Service (KWS) and the National Museums of various adaptations including its ability to survive in relatively high Kenya (NMK) for purposes of biodiversity research. For the other temperatures (approx. 36uC). The fish may therefore have locations outside protected areas, no special authorization was developed hypoxic resistance mechanisms since dissolved oxygen necessary. levels are generally low in warm waters. In addition, special As there is no Institutional Animal Care and Use Committee or adaptations may also be present that regulate sex determination equivalent animal ethic committee in Kenya, no formal approval mechanisms known to involve temperature [14,15]. This popula- was obtained for this work. To minimize suffering of individuals tion may in this regard potentially offer a model for the study of studied, specimens were captured using seine nets and immedi- sex determination in tilapine fishes. ately anaesthetized and killed using an overdose of MS-222 In a first study [5] samples from a single hot spring in the Loboi (Tricaine-S, Western Chemicals Inc). After death, a fragment of drainage, the Lake Bogoria Hotel Spring were analysed. At least muscle or fin tissue was taken from each specimen and two other springs with similar ecological conditions exist, each immediately preserved in 95% ethanol for later DNA analyses. hosting O. niloticus population, the Chelaba Spring that is located Voucher whole fish specimens were fixed in 4% formalin and later close to the Lake Bogoria Hotel Spring and the Turtle Spring preserved in 70% ethanol, and are presently curated at the located at the border of the papyrus marsh of Loboi Swamp. In National Museums of Kenya (NMK) in Nairobi. Other voucher order to protect these populations from various anthropogenic and specimens whose sequences were obtained from Genbank are environmental threats, there is an urgent need to characterise their curated at the Muse´e Royal de l9Afrique Centrale (MRAC) in natural genetic diversity as a first step of a future action plan for Tervuren, Belgium or at the Institut des Sciences de l9Evolution de sustainable management. The Loboi Swamp itself has receded in Montpellier (ISEM), France (Table 1). size by over 60% over a short period of 30 years due to expansion of irrigation via a ditch constructed in 1970 [13,16] and DNA extraction, sequencing and microsatellite
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